`wpmu_activate_signup()` returns an `array` or `WP_Error`. The array keys it returns can be different based on context of `$signup`. The return value was previously `extract()`'d, so the value of `$blog_id` was secretly undefined when being passed to `get_blogaddress_by_id()` in some cases.

If `$result['blog_id']` is not set, make sure `$url` is set to an empty string so that the proper "activated" message is shown to the user.

Props imath.
See [28373].
Fixes #29191.

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